1. Field of the Invention
The invention relates to systems for communicating data, and more particularly for transferring control parameters through the communications network used for transferring the data.
2. Description of the Related Art
In general, a communications network is connected to a set of terminal modules. The terminal modules are interconnected by the network, and data is transferred through the network, from the incoming fraction of the terminal modules then referred to as “incoming” terminal modules to the outgoing fraction of the terminal modules then referred to as “outgoing” terminal modules. These designations are used to facilitate understanding when referring to the direction of data transfer, it being understood that the terminal modules generally act simultaneously both as incoming terminal modules for data they transmit to the communications network and as outgoing terminal modules for data they receive from the network.
The terminal modules also exchange control parameters through the same communications network, typically for the purposes of supervising or controlling the operation of the communications system, i.e. the system comprising the network and/or its terminal modules. These control parameters are transferred in dedicated control messages.
By way of example, in an asynchronous network, a distributed mechanism for periodically verifying the consistency of connections established between terminal modules implies exchanging “information” parameters characterizing said connections. Under such circumstances, the information parameters are transferred in “information” messages which are transmitted by “source” terminal modules and received by “destination” terminal modules. Still by way of example, in an asynchronous switching network, a distributed dynamic inter-module rate-based flow control mechanism implies two types of mutual exchange of control parameters between all the pairs of terminal modules:                in the go direction, transferring “information” parameters characterizing broadband allocation requests for transmitting data through the network; and        in the return direction, transferring associated “response” parameters characterizing grants of authorized bandwidth.        
Under such circumstances, and throughout the description below, the term “go” direction is used to refer to transferring an information message from a “source” terminal module to a “destination” terminal module, and the term “return” direction is used for transferring an associated response message from said destination terminal to said source terminal.
The invention relates to the mutual transfer of such control parameters between terminal modules. In an asynchronous communications network, the corresponding dedicated control messages use inter-module control data blocks (packets or cells) which can advantageously be of varying lengths, such as multiple-segment cells, for example. FIG. 1 shows examples of multi-segment cells. The first cell shown is a three-segment cell; the first segment is marked SRT in the figure (for self routing tag) and constitutes the header for the cell, containing routing information; the following two cells may be used for placing the control parameters to be transmitted. FIG. 1 also shows another cell having five segments available for placing control parameters for transmission. Each segment typically has a size of 64 bits. The variable S is used below to designate the number of segments used for placing control parameters in a cell, such that the total size of a cell for a dedicated control message is 1+S segments, and its available payload capacity for placing control parameters is S times 64 bits.
A first conventional approach is based on point-to-point transfer of dedicated control cells from each terminal module to each other terminal module; such point-to-point messages can then contain only those parameters that relate to a particular pair of terminal modules. To simplify the wording of the text below, the term (information or response) “control parameter” is used in the singular when it relates to a given pair of terminal modules, it being understood that in practice this might involve a single parameter or else a group of parameters representing said pair.
Another conventional approach is based on point-to-multipoint transfer of dedicated control cells from each terminal module to each set of a given plurality (n) of terminal modules; such point-to-multipoint messages can then contain control parameters relating to a plurality (n) of pairs of terminal modules. In certain network configurations comprising a large number of terminal modules, those conventional solutions can give rise to relatively heavy traffic dedicated to control messages, which traffic is additional to the payload data traffic between terminal modules and can thus lead to traffic overloading in the network; one of the problems encountered is thus that of minimizing the impact of this additional traffic due to mutual transfers of control parameters.